(core) add appsumo endpoints with stub implementations

Summary:
This adds appsumo /token and /notification endpoints, with some
tests.  The stub implementation is sufficient for AppSumo
activation to succeed (when exposed via port forwarding for testing).
It needs fleshing out:

 * Implement upgrade/downgrade/refund and stripe subscription.
 * Implement custom landing page and flow.

Test Plan: added tests

Reviewers: dsagal, georgegevoian

Reviewed By: dsagal

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D2864

app/server/lib/GristServer.ts

@@ -20,6 +20,7 @@ export interface GristServer {
   getHomeUrl(req: express.Request, relPath?: string): string;
   getHomeUrlByDocId(docId: string, relPath?: string): Promise<string>;
   getDocUrl(docId: string): Promise<string>;
+  getOrgUrl(orgKey: string|number): Promise<string>;
   getResourceUrl(resource: Organization|Workspace|Document): Promise<string>;
   getGristConfig(): GristLoadConfig;
   getPermitStore(): IPermitStore;